JPH0215840B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention generally relates to a fuel assembly for a nuclear reactor;
More particularly, the present invention relates to a fuel assembly that allows for remote removal of selected fuel rods without disassembling the fuel assembly.

In many nuclear reactors, the core section includes a number of elongated fuel elements or rods (called fuel assemblies) grouped within and supported by a skeletal structure. The fuel assemblies are generally vertically elongated and are supported and aligned by upper and lower core support plates that extend laterally. In the most common structure, the axis of the core support tank extends vertically, and a number of fuel assemblies are also vertically arranged and rest on a lower core support plate. A conventionally configured fuel assembly has a plurality of fuel rod and control rod guide thimbles or guide tubes that are secured to the control rod guide thimbles and spaced apart along the fuel assembly. A lattice is placed to hold them in organized rows. Opposite ends of the fuel assembly, upper and lower nozzles, are secured to a control rod guide thimble to form an integral fuel assembly. The upper and lower nozzles extend slightly above and below the ends of the fuel rod, respectively, and hold the fuel rod therebetween.

When starting up a new or prototype nuclear reactor, it is common practice to periodically inspect and inspect selected fuel assemblies after a certain period of operation. In addition to inspecting damaged fuel rods, checking for wear on fuel assemblies and deterioration of the fuel rods themselves, some fuel rods may be removed from others if a higher enrichment of fissile material is desired. It will be replaced with a different type of fuel rod.

Typically, during operation of a nuclear reactor, the fuel assemblies in the core region are immersed in water or other suitable coolant to a depth of approximately 10 meters, with each fuel assembly being separated into organized rows. There is a tight fit between adjacent fuel assemblies that make up the fuel assembly. Therefore, in order to carry out inspection and inspection operations such as those described above, the selected fuel assembly is generally lifted from its working position and either completely removed from the reactor or moved to a suitable location. The product is brought to the work area, where necessary inspections or inspections are performed. During this time, the fuel assembly is immersed in the cooling water to a depth of about 2 m. However, if one of the fuel rods is found to be damaged or replacement or substitution with a fuel rod of a different composition is desired, conventionally the entire fuel assembly is scrapped and A new fuel assembly was installed in its place.

In other words, if we know representatively the state of some fuel rods in a fuel assembly in a nuclear reactor, we can predict the state of all the fuel rods in that fuel assembly. The decision can be made as to whether or not to remove the fuel rods from the reactor, but conventional fuel assemblies were not configured to allow removal of some of the fuel rods in the reactor to check their condition. The fuel assembly was removed from the reactor and the necessary checks were performed.

Therefore, in view of the high costs associated with the replacement of fuel assemblies, it is an object of the present invention to allow inspection or maintenance of the fuel assemblies in situ, i.e. within the reactor, without disassembling the fuel assemblies or removing them to another location. It is an object of the present invention to provide an improved fuel assembly that can be used during start-up of a new or prototype nuclear reactor in which a limited number of fuel rods can be removed and replaced remotely for checking purposes.

To this end, the present invention includes an upper nozzle, a lower nozzle, at least one control rod guide thimble extending upwardly from the lower nozzle, and a plurality of transverse gratings spaced apart along the control rod guide thimble. and an organized row of elongated fuel rods supported by the lattice in parallel spaced relation to each other and axially constrained between the lower nozzle and the upper nozzle. and the upper nozzle includes a laterally extending adapter plate supported on the upper end of the control rod guide thimble at a level above the upper ends of the fuel rods, the adapter plate having a plurality of coolant flow openings therein. a monolithic construction of a plurality of coupling portions interdigitated with each other in an intersecting manner to define and arranged in overlapping relationship with respect to a majority of the fuel rods of the organized row; and within the area of the adapter plate; at least one plug means incorporated into the plate and disposed above the remainder of the organized row of fuel rods;
The plug means is disposed between and attached to adjacent ones of the coupling portions, and is adjacent to and connected to at least one fuel rod immediately below the plug means. a retaining portion having an axial throughbore of sufficient diameter to permit removal of the fuel rod; and a retaining portion having a cross-sectional dimension that is seated and removably retained within the throughbore and prevents passage of the fuel rod. and a plug portion having at least one coolant passage extending axially therethrough.

According to a preferred embodiment, the plug means, ie the cylindrical plug part of the access plug, has a male thread and the axial hole (through hole) of the holding part has a female thread, so that the plug part is fixed to the plug part by a threaded connection. It can be so. In order to prevent the access plug from oscillating diffusely, locking means in the form of rising tongues are provided on the plug part, these tongues being bent above the recesses in the upper surface of the retaining part in the vicinity of the axial bore. to be engaged. A through shaft hole is formed in the plug part to allow coolant to flow through it.
The shaft hole is sized to prevent movement of a single fuel rod upwardly into the plug section.

Furthermore, the upper nozzle in a preferred embodiment is
A retainer plate is provided axially spaced above the adapter plate. The hold-down plate has at least one coolant flow opening, and the flow opening is positioned above the plug portion to allow one fuel rod to pass through the fuel assembly when the plug portion is removed from the holding portion. It is attached to the upper end of the guide thimble so that it can be taken out upwards to the outside. This coolant flow opening in the holding plate preferably has a larger cross-sectional area than the coolant flow opening in the plug portion.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG.

In the figures, the same numbers represent the same or corresponding parts. Also, "forward", "backward",
Terms such as "left side", "right side", "upper", "lower", etc.
It is used merely for convenience and should not be construed as limiting.

In the drawings, particularly in FIG. 1, the fuel assembly 10 illustrated therein includes a lower end structure or lower nozzle for supporting the fuel assembly 10 on a lower core plate in the core region of a nuclear reactor (not shown). 12, a plurality of control rod guide tubes or guide thimbles 14 extending longitudinally within the fuel assembly projecting upwardly from the lower nozzle 12, and a plurality of lateral grids spaced axially along the guide thimbles 14. 16 and systematic rows of elongated fuel rods 18 (only one shown in FIG. 1) laterally spaced from one another and supported by a lattice 16. According to this embodiment, the lower ends of the long fuel rods 18, although shown axially held and spaced above the lower nozzle 12, rest on the upper surface of the lower nozzle 12. It may be supported so that The instrumentation tube 20 is arranged at the center of the fuel assembly 10. A top structure or top nozzle 22, designated generally by the numeral 22, is secured to the top end of the guide thimble 14 and forms an integral fuel assembly that can be handled in the normal manner without damaging its components. is forming.

The transverse grate 16 is secured to the longitudinally extending guide thimble 14 at axially spaced predetermined locations during the formation of the fuel assembly 10;
The fuel rods 18 are inserted from below through the grid 16, after which the lower nozzle 12 is suitably secured to the lower end of the guide thimble 14, for example by a machine screw 24, and finally the upper nozzle 22 is inserted into the guide thimble 14. It is fixed to the upper end. Control rods (not shown) that control the nuclear fission process can reciprocate within the control rod guide thimble 14 of the fuel assembly 10.

The illustrated fuel assembly 10 is shown as having square rows (19×19) of fuel rods 18 with sixteen control rod guide thimbles 14 strategically placed within the rows of fuel rods. Further, both the lower nozzle 12 and the upper nozzle 22 have a generally rectangular cross-section, and each control rod guide thimble 14 has a separate guide thimble extension that extends from the upper end of the associated guide thimble 14. and is adapted to form a normal extension thereof. Each guide thimble extension, hereinafter simply referred to as a thimble extension 26, is coaxial with and has a diameter of approximately the same size as the corresponding guide thimble 14, and is connected by any suitable method such as a bulge fit. is preferably connected to the upper end of the guide thimble 14. However, the invention is not limited to any particular nozzle shape, number or shape of fuel rods and guide thimble, or number and shape of thimble extensions.

According to a preferred embodiment, the upper nozzle 22 includes:
Illustrated as a subassembly including a hold-down device, the hold-down device holds the fuel assembly 10 in direct contact with the top surface of the upper core alignment support plate.
It can be placed in the core region to provide a holding force that prevents the fuel assembly from being lifted up by liquid while the coolant is flowing upward, and to reduce the length of the fuel assembly due to thermal expansion or other factors induced in the core. Allow for change.

As can be seen in FIG. 1, the upper nozzle 22 subassembly includes a coil spring 28 disposed about the upper end of each thimble extension 26, and the coil spring 28 is coupled to the lower adapter plate 30. It is arranged between the upper pressing plate 32 and the upper pressing plate 32 . The lower adapter plate 30 is slidably mounted on the thimble extension 26 by a through hole 34 and downward movement of the adapter plate 30 along the thimble extension 26 is limited by a retaining member 36. A retaining member 36 is attached to each thimble extension 26 at an axially spaced location above the upper end of the fuel rod 18.
One is attached to the top. adapter plate 30
has an integral elongate strip-shaped connecting portion 38 (see FIGS. 2 and 3) interlaced with each other in a cross-shaped manner defining a plurality of coolant flow openings 40 for the coolant, and one portion of the connecting portion 38 is located and oriented on the thimble extension 26 such that the fuel rod 18 is above the upper end of the fuel rod 18 . When the adapter plate 30 is positioned above the thimble extension 26 , it prevents upward movement of the fuel rods 18 through the flow opening 40 and thus prevents the fuel rods 18 from moving between the upper nozzle 22 and the lower nozzle 12 . axially restrained or restrained.

The upper hold-down plate 32 is slidably mounted on the thimble extension 26 by respective passageways 43 (FIG. 1), each passageway 42 defining a shelf 44 therein. The hold-down plate 32 also has a number of relatively large coolant flow apertures 46 (FIG. 2) and a number of relatively small coolant flow apertures 48 disposed at predetermined locations thereof. A separate retaining member 50 spaced axially above each retaining member 36 (FIG. 1) includes a retaining member 50 for each thimble extension 26.
and are located within the respective passages 42. The holding member 50 has an inner shelf portion 44
cooperates to limit upward movement of the hold-down plate 32 along the thimble extension 26. A raised side wall 52 is formed at the peripheral edge of the lower adapter plate 30 to form an enclosure guiding the upward flow of coolant in the upper region of the fuel assembly 10 . It should be noted that the fuel rod removal means to be described is not limited in its use to the upper nozzle subassembly described above and shown in the drawings.

According to the invention, the fuel assembly 10 remains within the core region of a nuclear reactor (not shown) without disassembling the fuel assembly 10 (e.g. by removing the upper nozzle 22 from the fuel assembly 10). means for removing selected ones of the fuel rods 18 from the fuel assembly 10. More specifically, as shown in FIGS. 2 and 3, the fuel assembly 10 has a pair of approach plugs (plug means),
Each access plug has a retaining portion 54 and a plug portion 56 and is confined within the lower adapter plate 30 of the upper nozzle 22 and is specifically positioned above a preselected one of the fuel rods 18. (Figure 3).

The retaining portion 54 of each access plug has a generally cruciform shape with concave corners and lies between the coupling portions 38, as best shown in FIGS. The retaining portion 54 connects its side to the adjacent connecting portion 38.
By properly fixing it, for example by welding,
It is fixed between the connecting parts 38. To position the retaining portions 54 within the adapter plate 30, predetermined sections or portions of the coupling portions 38 may be cut and removed, and a physical It forms a space and space. In a preferred embodiment, the retaining portion 54
is provided with a through shaft hole (through hole) 58 having a diameter sufficient to approach and remove the four fuel rods 18. Preferably, the shaft hole 58 has a female thread.

The plug portion 56 (best shown in FIG. 5) of each access plug is in the form of a cylindrical body that fits within the axial bore 58 and preferably has external threads 60 for engagement with internal threads in the wall of the axial bore 58. is prepared. Four pie-shaped passages 62 extend axially through the plug portion 56 to permit upward flow of coolant therethrough, the cross-sectional dimensions of the passages 62 being such that the fuel rods It is sized to prevent it from extending and passing through the passageway.

In order to ensure that the plug part 56 is screwed and fixed in the shaft hole 58 of the retaining part 54 and does not loosen due to vibration or otherwise during operation of the fuel assembly 10, a raised tongue 64 and a recess 66 are provided. A locking means in the form of is provided. A tongue piece 64 is formed on the upper surface of the plug portion 56 and the retaining portion 54
It is bent over a corresponding recess 66 formed in the upper surface of the holder and is adapted to engage and lock therein. Recess 66 extends radially outward from shaft hole 58 .

As best shown in FIG. 2, large coolant flow openings 46 are located in the upper hold-down plate 32, one flow opening 46 is located above one of the access plugs, and the other flow opening 46 is located above one of the access plugs. is formed so that it is above the other one. Because of this positional relationship or alignment, the plug portion 5
6 and remove it from the holding part 54, grasp the upper ends of the four lower fuel rods 18 and remove them from the fuel assembly 10 through the shaft holes 58 of the holding part 54 and the coolant flow openings 46. Fuel rods can be lifted vertically upwards. Such fuel rod removal and subsequent replacement in a similar manner can, of course, be carried out while the fuel assembly remains in the core region and without removing the upper nozzle.

[Brief explanation of drawings]

FIG. 1 is an elevational view, partially in cross section and partially cut away, of a fuel assembly in which the present invention is implemented;
FIG. 2 is an enlarged plan view of the fuel assembly of FIG. 1 showing two access plugs attached to the upper nozzle adapter plate; FIG. 3 shows one access plug in place and the other The access plug is removed and the four individual fuel rods are exposed for removal, with cross-sectional views taken along line 3--3 of FIG. 1 and FIG.
FIG. 5 is an enlarged perspective view of the plug holding portion removed from the upper nozzle. FIG. 5 is a perspective view of the removable plug portion after being removed from the upper nozzle. 10...Fuel assembly, 12...Lower nozzle, 1
4... Control rod guide thimble, 16... Lattice, 18
... fuel rod, 22 ... upper nozzle, 30 ... adapter plate, 38 ... joint part, 46 ... circulation opening,
54... Holding portion (plug means), 56... Plug portion (plug means), 58... Shaft hole (through hole).

Claims (1)

[Claims] 1 an upper nozzle, a lower nozzle, at least one control rod guide thimble extending upwardly from the lower nozzle, a plurality of transverse grids spaced apart along the control rod guide thimble, and spaced parallel to each other; an organized row of elongated fuel rods supported by the lattice in a symmetrical relationship and axially constrained between the lower nozzle and the upper nozzle, the upper nozzle being , a laterally extending adapter plate supported on the upper end of the control rod guide thimble at a level above the upper end of the fuel rods, the adapter plates intersecting each other to define a plurality of coolant flow openings. a monolithic construction of a plurality of coupling parts interlaced and arranged in overlapping relationship with respect to a majority of the fuel rods of said organized row; at least one fuel rod disposed above the remainder of the organized row of fuel rods;
said plug means being disposed between and attached to mutually adjacent ones of said coupling parts, and said plug means being disposed between and attached to said adjacent ones, said plug means said at least one fuel rod immediately below said plug means. a retaining portion having an axial through-hole of sufficient diameter to permit access to and removal of the fuel rod;
a plug portion mounted and removably retained within the through-hole and having at least one coolant passage extending axially therethrough and having a cross-sectional dimension to prevent passage of fuel rods; Fuel assembly for nuclear reactor.